As is known, biological sewage purification plants have been provided wherein the water to be treated is aerated in a tank containing bacteria by means of an aerating device which is preferably arranged in the region of the liquid level in the tank. The sewage is drained from the tank after a certain period of time during which it is aerated and exposed to the action of the bacteria. The oxygen necessary for maintaining and multiplying the strain of bacteria is supplied by means of an aerating device which preferably consists of a rotating impeller which constantly swirls the surface of the water to be treated such that it is enriched with air which permeates the entire contents of the tank. Other types of aerating systems, however, may also be employed.
In the operation of a sewage purification plant of this type, it is necessary to hold the temperature in the aerating tank at the optimum point which insures the best level of decomposition by the bacteria, irrespective of the external temperature which happens to prevail and of the inlet temperature of the water to be treated. If this temperature is not maintained, a considerable reduction in efficiency of decomposition occurs, particularly when the external temperature drops markedly. On the other hand, when the temperature is too high, efficiency may decline to almost a zero value.
In the past, proposals have been made for arranging heat exchange surfaces in the aerating tank itself or for introducing direct steam in order to heat up the contents of the tank when the external temperature falls. It has been found, however, that with heat exchange surfaces or direct introduction of heat, the temperature-sensitive bacteria, which come into contact with the heat exchange surfaces or the steam, will die. Also, it has been found necessary to subject the heat exchange surfaces to frequent cleaning due to heavy deposits of sludge with the overall result that this method has proved to be impractical.
Heating the incoming sewage and/or the return sludge before entry into the tank has likewise proved to be unsuitable because of local overheating inside the tank or at the heat exchange surfaces which results in the death of the bacteria. In addition, rather large amounts of sewage must be heated and supplied to the tank to maintain the desired temperature. Such large amounts of sewage do not insure the proper dwell time of the sewage in the tank to facilitate bacterial growth. Furthermore, radiation losses at the surface of the water are too high.